In various fields including the industry, household electric appliance, and automobile, small-sized, high-efficiency three-phase synchronous motors are widely used. Generally, in a three-phase synchronous motor, the rotational position of the rotor having magnets is detected by magnetic detection element such as a hall IC. Based on the detection result, the armature coils on the stator side are excited sequentially to rotate the rotor. Some motors can be driven by sinusoidal current with the use of accurate rotational position detectors such resolvers, encoders, and GMR sensors, thereby reducing the oscillation or noise of a torque ripple.
However, if such a rotational position detector becomes defective, the three-phase synchronous motor ceases to rotate. This is true of resolvers, encoders, and GMR sensors used as rotational position detectors. Since failure of rotational position detectors leads to malfunction or abnormal operation in the case of the drive device for a three-phase synchronous motor such as an electric power steering device, improvements have been called for.
The invention disclosed in Patent Document 1 has, in addition to a rotational position detector, rotational position estimating means for estimating the rotational position of the rotor from the voltage and current of the three-phase synchronous motor when the rotational position detector is defective. The output of the rotational position estimating means is used in place of the output of the rotational position detector, whereby the three-phase synchronous motor is driven stably at the time of failure of the rotational position detector.